Synthesized high polymer latexes are extensively used. If a given latex happens to contain an unreacted monomer, the odor, toxicity and other adverse factors due to the presence of the unreacted monomer pose a serious problem from a hygienic point of view. Particularly, when the latex is used as the raw material for food packages or as the raw material for products destined to come into direct contact with the human body, it is important that the latex be free from residual unreacted monomer. It becomes necessary, therefore, that such a latex be separated from the residual unreacted monomer.
Various methods have heretofore been proposed for the removal of unreacted monomer from the latex. In one prior method nitrogen, air or some other similar gas is directly blown into the latex. Another prior method employs an agitated thin film evaporator and another utilizes a packed column. The method which involves direct contact between nitrogen, air and the latex, however, is undesirable in that the treatment is time-consuming and removal of the unreacted monomer is difficult to accomplish and therefore, the efficiency of the operation of removal is low. The method which involves use of an agitated thin film evaporator requires forced formation of a film of latex by means of agitation and therefore, tends to induce coagulation of latex and vigorous growth of bubbles in the latex. In the method which resorts to the use of a packed column, solid components contained in the latex tend to separate and deposit, for example, on the inner wall surface of the column and on the surface of packing in the column possibly because of nonuniformity in the wetting of the column interior by the latex, and the column itself is extremely difficult to clean which is required for maintenance. Thus, none of the methods mentioned above prove satisfactory. In addition to the methods described above, various other methods have been developed in accordance with particular application in which the latex is to be used. When a latex is used as a coating agent, for example, the removal of residual unreacted monomer is effected by vaporizing the unreacted monomer while the coating film of latex is being dried. When a latex is treated so as to separate and recover solid components from the latex which are used in the form of a powder, the removal of unreacted monomer can be accomplished by desiccating the solid components after their recovery from the latex. Frequently, such drying treatments fail to provide thorough removal of the unreacted monomer. Moreover, these treatments in many cases involve hygienically undesirable phenomena such as emanation of an objectionable odor.
As described above, various methods have been adopted to date for the removal of unreacted monomers from latexes. By these methods, however, thorough removal of unreacted monomers from latexes can not always be accomplished satisfactorily even from an economical point of view. Moreover, the present state of technology does not enable such unreacted monomers to be thoroughly recovered and put to reuse.